The invention relates generally to the detection of halogenated hydrocarbons, and more particularly to the fluorometric and/or colorimetric detection of gem-polyhalogenated hydrocarbons.
Organohalogens are used extensively in medicine, industry, and agriculture worldwide. Some classes of these compounds are known to be carcinogenic, teratogenic, or otherwise toxic. Because of their widespread use and pervasiveness in the environment, concern has arisen as to the possible deleterious effects that unintended exposure to them might have on human health, Horvath, Halogenated Hydrocarbons (Marcel Dekker, Inc., New York, 1982).
The industrial and commercial applications of organohalogen compounds are almost innumerable. They are used as solvents for the extraction of natural products, dry cleaning fluids, degreasing agents, fuel additives, fumigants, and intermediates for the synthesis of a multitude of other organic compounds. Polychlorobiphenyls (PCBs) are used as dielectric fluids in the construction of transformers, and chlorofluorocarbons are used as refrigerants and also as aerosol propellants for the dispersal of a multitude of household products. Many of these compounds are known carcinogens, Kraybill, et al. Eds, Environmental Cancer (John Wiley & Sons, New York, 1977).
Chloroform, first synthesized in 1831, was used as a general anesthetic in 1847, and it and other organic compounds provided the sedation and relief from pain needed for the development of modern surgery. Chloroform has now been replaced by safer and more effective halogen relatives such as halothane, methoxyfluorane, enflurane, and the like. Unfortunately, chronic exposure to such agents, e.g. among anesthesiologists and other health care personnel, is correlated with higher rates of miscarriages, and psychomotor impairment.
Recently, the specter has arisen that organohalogen compounds formed inadvertently in the environment could be carcinogenic or otherwise toxic. About 1.1 million tons of chlorine are used annually for water purification. Water, of course, contains many organic impurities, some of natural origin and others anthropogenic. Inevitably some of these are chlorinated during treatment, which leads to the introduction into drinking water of a vast miscellany of organohalogen compounds, some of which are known to be carcinogenic, such as trichloroethylene. Other organohalogens are introduced into public water supplies, particularly aquifers, as pollutants by accidental release or mishandling of a variety of organic solvents used in industry.
Concern about the health effects of organohalogens in the environment has lead to the development of methods for detecting and monitoring their presence.
One such method is based on the reaction of halogenated hydrocarbons with pyridine or pyridine derivatives in an alkaline medium to yield highly colored products. It is known in the art that when a gem-polyhalogenated compound is heated with pyridine in a strongly alkaline medium, such as in the presence of sodium or potassium hydroxide, a product forms which is both chromogenic and fluorescent.
This reaction scheme, shown in Equation I below, ##STR1## is known as the Fujiwara reaction (K. Fujiwara, Sitzfer, Abhandl. Naturforsch. Ges. Rostock., 6, 33, (1914); G. A. Lugg, Anal. Chem., 38, 1532 (1966); T. Uno et al., Chem. Pharm. Bull., 30, 1876 (1982). The Fujiwara reaction has become the classical method for the detection of halogenated hydrocarbons in the liquid phase. However, for a quantitative measure of halogenated hydrocarbons in a test solution or sample, the Fujiwara reaction presents some problems, due to the insolubility of pyridine in reagents normally used to generate the necessary alkalinity, and the difficulty in being able to control the rate of diffusion of the OH-- ion from the aqueous phase into the organic pyridine phase.
More often, the reaction consists of a two-phase procedure whereby the gem-polyhalogenated compound, pyridine and aqueous sodium or potassium hydroxide are combined, mixed, and heated for a predetermined length of time until an intensely red color develops, which is due to the chromogenic product. The pyridine phase is then separated from the alkaline phase by conventional methods. Absorption spectra of the colored product are measured thereafter. The amount of the chromogenic product formed is dependent on the amount and rate of diffusion of the hydroxide ion into the pyridine phase of the mixture as well as the concentration of gem-polyhalogenated hydrocarbon in the sample solution. Since this diffusion is difficult to control, reproducibility of measurements becomes extremely difficult and unpredictable.
A one phase procedure using pyridine-water-sodium hydroxide has also been employed to avoid the pitfalls of the two phase method (G. A. Lugg, supra). However, current single phase procedures result in much lower sensitivity because only a small fraction of the single phase reaction mixture can consist of water, which is required for creating the necessary basicity from dissolved alkali metal hydroxides. The lower concentration of hydroxyl radicals results in lower amounts of chromogenic indicator for detecting the presence of gem-polyhalogenated hydrocarbons.
Apart from the particular analytical method used to detect organohalogens in a liquid sample, additional problems exist with currently used methods for monitoring groundwater contaminants. Typically such methods now involve digging well fields having numerous boreholes large enough to admit sample collectors, which are then brought back to a laboratory for analysis, e.g., Young and Baxter, "Overview of Methods for Ground Water Investigations," in Ward, et al., Eds., Ground Water Quality, pgs. 219-240 (John Wiley & Sons, New York, 1981).
Fiber optic sensing devices currently being developed offer a way to avoid the expense associated with currently available monitoring systems, e.g. Hirschfeld, et al, "Feasibility of Using Fiber Optics for Monitoring Groundwater Contaminants," Optical Engineering, Vol. 22, pgs. 527-531 (1983). In particular, many miniature in situ optical probes can be distributed throughout the region to be monitored, and connected by fiber optics to a centrally located instrument for spectroscopic analysis. Thus remote real-time continuous monitoring is possible with such a system.
Miller, et al, in co-pending U.S. patent application Ser. No. 721,150, filed Apr. 8, 1985, now U.S. Pat. No. 4,666,672 issued May 19, 1987, disclose a fiber optic sensor which uses a two phase Fujiwara reaction for detecting halogenated hydrocarbons. In the preferred embodiment the two phases are held contiguously inside a capillary tube attached to the end of a fiber optic such that an interface between the two phases is formed adjacent to the end of the fiber optic.
The position of the end of the fiber optic with respect to the interface is critical to the reliable and repeatable operation of the sensor. However, because the aqueous phase has a different density than the organic phase, any change in position of the capillary tube causes the interface to change position with respect to the end of the fiber optic which, in turn, unpredictably alters the optical signal collected by the fiber optic.
Clearly it would be advantageous to have an optical probe for detecting halogenated hydrocarbons based on the Fujiwara reaction which is not limited by the problems associated with a two phase reaction system.